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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        class Father{
            constructor(x,y){
                this.x = x;
                this.y = y;
            }
            sum(){
                console.log(this.x + this.y);
            }
        }
        //子类继承父类加法方法  同时 扩展减法方法
        class Son extends Father{
            constructor(x,y){
                //调用父类的构造函数
                super(x,y);//super()必须在子类this调用之前调用
                this.x = x;
                this.y = y;
                
            }
            subtract(){
                console.log(this.x - this.y);
            }
        }
        var son = new Son(5,3);
        son.subtract();
        son.sum();
    </script>
</body>
</html>